The present invention relates to multistage amplifying thyristors with gate and main stages and, more particularly, relates to a multistage amplifying thyristor in which the peak turn-on current in the gate and any intermediate thyristor stages is limited during thyrisitor turn-on.
Conventional multistage amplifying thyristors include gate and main stages and may include one or more intermediate stages. The reason for including several stages in a thyristor is to permit turn-on of the thyristor with a gate signal of very low energy. For example, a light triggered thyristor (LTT) must rely upon a minute amount of light energy (typically on the order of about 20 nanojoules) to turn on the thyristor. This is possible since the light energy turns on only a gate thyristor stage which is highly-sensitive but of low current rating and which, in turn, turns on any intermediate thyristor stage. Successive turn-on of the various thyristor stages continues until turn-on of the main thyristor stage is turned on.
A significant limitation of conventional multistage amplifying thyristors is that the rate of turn-on, or di/dt, of such thyristors used in typical circuits must be controlled by external circuit devices in order to prevent thermal stresses in the thyristor from destroying the thyristor. For instance, a typical high voltage direct current tansmission system utilizing thyristors for alternating current-to-direct current conversion incorporates relatively expensive saturable reactors. This type of reactor presents a temporary, high inductive impedance to turn-on current flow in a thyristor, but rapidly falls off in its level of inductive impedance upon steady state thyristor operation.
A published approach to designing a thyristor with enhanced immunity from di/dt thermal stress failure, known as the controlled turn-on approach, is to incorporate into a multistage amplifying thyristor current control resistor regions, one between each pair of adjacent thyristor stages. These current control resistor regions are intended to reduce turn-on, or di/dt, thermal stress in a thyristor by functioning to, first, reduce the current in each preceding or prior turned-on thyristor stage and, second, reduce the duty cycle of each preceding thyristor stage. The subject approach is described in detail, for example, in an article by VAK Temple (the present inventor) entitled "Controlled Turn-on Thyristors" published in IEEE Transactions on Electron Devices, Vol. ED-30 (July 1983) at pages 816-824, the entirety of this article being incorporated herein by reference.
The foregoing Temple article describes successful performance of 5 kilovolt thyristors that were tested to a 600 volt level. Subsequent testing of thyristors such as described in the Temple article by the present inventor has shown the occurrence of turn-on failure, due to thermal stresses, at about the 2000 volt level. Further investigation into the controlled turn-on approach described in the Temple article has revealed that certain considerations, not taken into account in the investigation described in the article, are a factor in causing turn-on failure at about the 2000 volt level. The present invention addresses these considerations and results in a multi-stage amplifying thyristor successfully exhibiting controlled turn-on at considerably higher voltages than heretofore attainable.
In general terms, the considerations not addressed in the Temple article are centered around the decrease in resistance value of the current control resistor regions incorporated into a thyristor, due to what is known in the art as a "modulation" effect in such resistor regions. It is important to note that these resistor regions comprise semiconductor material of nominal doping concentration and of either P- or N-conductivity type. The increase in either majority or minority carrier concentration in the current control resistor regions results in modulation, or lowering, of the resistance of such regions. This is readily appreciated in the case of increased majority carrier concentration due to the overall increase in concentration of majority carriers; however, in the case of increased minority carrier concentration, an additional phenomenon known in the art as the principle of quasi-neutrality is involved. In accordance with this principle, the concentration of majority carriers in a semiconductor region increases to roughly that of the minority carrier concentration, preventing unduly high electric fields from occurring in the thyristor.
The Temple article points out that within a multistage amplifying thyristor, there are sources of mobile carriers (in particular, the cathode emitter layers of the various thyristor stages) that increase the current carrier level in the current control resistor regions, unless these regions are sufficiently spaced or otherwise shielded from such sources of carriers. The present invention is directed to a thyristor design in which further sources of mobile carriers, not recognized at the time of publication of the Temple article, are spaced or shielded from such sources of mobile carriers by an extent sufficient to minimize modulation of the current control resistor regions.